During the course of this work, we have found that pretreatment of human monocytes with M-CSF results in an inhibition of the release of IL-1 from LPS stimulated human monocytes. Northern blot analysis revealed that M-CSF does not alter the LPS induction of IL-1 message. In addition, immunoprecipitations performed using an antibody which binds both the pro and processed forms of IL-1, indicate that the message is translated into intracellular IL-1. However, it appears the intracellular protein is not effectively released from the cells. We plan to use these findings to examine the events involved in IL-1 release from human monocytes and to determine, by use of immunohistochemical staining whether all monocytes stimulated with LPS produce IL-1 or whether only a subpopulation of monocytes is capable of IL-1 production.